Flexible printed wiring boards (FPCs) such as coverlays and metal clad laminates have conventionally been used, the flexible printed wiring board being formed by properly combining a resin layer having insulation such as a polyimide film and a polyamide film, an adhesive layer including an epoxy resin or a polyimide resin as a principal component, and a metallic foil layer having conductivity such as a copper foil, a silver foil, and an aluminum foil.
As the metal clad laminate, a three-layer flexible metal laminate formed of a metal layer, a polyimide layer, and an adhesive layer such as an epoxy resin, and a two-layer flexible metal laminate formed of a metal layer including a metallic foil and a polyimide layer are known.
Recently, the flexible metal clad laminate has often been used for electronic and optical apparatuses. Following this, light blocking properties of the flexible metal laminate are increasingly demanded. For example, in a method for brightening a mobile display, there are into two types: a planar light source type, and a type in which a light source is provided in an edge of a display and the whole display surface is brightened by a light guide plate and the like. Elaborate calculation is performed and both types are designed in order to brighten the display efficiently. However, there is a problem of both types of the planar light source and the edge light source that the light leaks from a flexible printed wiring board incorporated around the display, resulting in leak of the light to the outside from a junction of a casing. As a technique for solving this problem, a technique for giving a black color to a polyimide layer by blending of a pigment that absorbs light of all wavelengths is known.
For example, JP 9-135067 discloses a light-blocking coverlay film containing a black pigment having a specific average particle size in one or both of an insulation film and a thermosetting adhesive.
JP 6-350120 discloses a flexible printed wiring board in which one or both of an insulating substrate and a coverlay are a para-oriented aromatic polyamide film containing not less than 1% by weight of a dye or a pigment.